Inhibition of polyurethane hardening

ABSTRACT

Elastomeric polyurethanes, e.g. as prepared by reacting diisocyanate-capped, essentially linear polyester or polyether and polyol suitable for cross-linking thereof, are inhibited against undesired hardening in use or storage by including in the polyurethane-forming reactant mixture a plasticizing composition composed of carboxylate having a 200-1,000 molecular weight and at least two monocyclic aryl radicals per molecule and, based on the weight of said carboxylate, 0.1-5% glycol having a 62-500 molecular weight.

BACKGROUND OF THE INVENTION

It is well known to make elastomeric polyurethanes for various uses suchas, e.g., roller covers for printing or other presses, impact-resistantcoverings, gaskets, seals, etc. Typically, such polyurethanes can beproduced by reacting essentially linear polyester or polyetherprepolymer with at least one glycol or polyol suitable for cross-linkingof the prepolymer, e.g. as described in U.S. Pat. No. 3,769,265 issuedOct. 30, 1973 to G. Grogler et al. As disclosed in that patent,plasticizers such as phthalic acid esters can be added in thepreparation of such polyurethanes.

It has been found, however, that in extended storage or end use of manyelastomeric polyurethanes, the polymer tends to harden, commonly to adegree which substantially impairs or even eliminates suitability of thepolymer for its intended use as an elastomeric material. Hence it ishighly desirable to provide a technique for inhibiting such hardening,and it is an object of this invention to provide such a technique.Another object is the provision of a polyurethane-plasticizingcomposition which inhibits undesired hardening of the polyurethane.Another object is an improved elastomeric polyurethane compositionhaving an inhibited tendency toward such hardening. Other objectsinclude improvements in the preparation of such polyurethanes wherebythe resulting polymer is inhibited against such hardening. Additionalobjects will be apparent from the following disclosure in which allparts and percentages are by weight except where otherwise noted.

SUMMARY OF THE INVENTION

In accordance with this invention, it has been found that theaforementioned undesired hardening of elastomeric polyurethanecompositions can be substantially and, in some cases, virtuallycompletely inhibited by including in the mixture of reactants used toprepare the polyurethane a composition consisting essentially ofpolyurethane-plasticizing carboxylate having a molecular weight fromabout 200 to about 1,000 and at least two monocyclic aryl radicals permolecule and, based on the weight of said carboxylate, from about 0.1%to about 5% of at least one glycol having a molecular weight from 62 toabout 500.

Thus, an elastomeric polyurethane composition substantially inhibitedagainst such hardening can be prepared by subjecting topolyurethane-forming reaction conditions an elastomericpolyurethane-forming reactant mixture comprising (a)diisocyanate-capped, essentially linear polyester or polyether, (b)polyol suitable for cross-linking of said polyester or polyether toprovide an elastomeric polyurethane, (c) polyurethane-plasticizingcarboxylate having a molecular weight from about 200 to about 1,000 andat least two monocyclic aryl radicals per molecule and, based on theweight of said carboxylate, from about 0.1% to about 5% (d) glycolhaving a molecular weight from 62 to about 500.

Correspondingly, in a process for preparing such an elastomericpolyurethane, this invention provides an improvement which comprisesinhibiting the polyurethane against undesired hardening by including inthe mixture of polyester or polyether and polyol reacted to form thepolyurethane a carboxylate as just described and, based on the weight ofsaid carboxylate, from about 0.1% to about 5% glycol of the kind justmentioned.

DETAILED DESCRIPTION OF THE INVENTION

As used herein regarding polyurethane-plasticizing compositionsconsisting essentially of carboxylate and glycol, the term "consistingessentially" means that such compositions contain no other constituentwhich substantially alters the effect of the carboxylate and glycol onhardness stability of an elastomeric polyurethane produced with use ofsuch a composition as described herein.

The polyurethanes to which this invention pertains include any of thevarious well-known solid, elastomeric varieties of the "low-durometer"kind, i.e., those characterized by a Shore A Hardness (measured inaccordance with ASTM Method D 676-58T) from about 10 to about 100 andeven more typically from about 20 to about 70. As is known in the art, agreat variety of monomers can be used in the preparation of suchpolyurethanes. Normally, there is employed a prepolymer which is usuallyan essentially linear polyester, for instance the reaction product of aglycol such as an alkylene (e.g. α,ω-alkylene) glycol, preferablycontaining from two to about six carbon atoms per molecule, and adicarboxylic acid such as an alkylene (e.g. α,ω-alkylene) dicarboxylicacid, preferably containing from about four to about eight carbon atomsper molecule, or an essentially linear polyether, for instance a polymerof a glycol such as an alkylene (e.g. α,ω-alkylene) glycol. Typically,the polyester or polyether is end-capped with a multifunctionalisocyanate, for instance an arylene diisocyanate such as, e.g., toluenediisocyanate, or an alkylene diisocyanate such as, e.g., hexamethylenediisocyanate. By essentially linear is meant that the polyester orpolyether has a relatively low degree of branching such that the averagenumber of isocyanate end groups per molecule is from two to about sixand preferably not more than about four. Average molecular weight ofsuch an end-capped polyester or polyether is conventionally from about500 to about 20,000 and even more typically from about 1,000 to about6,000.

In accordance with the invention disclosed herein, preparation of thepolyurethane is effected by reacting such a prepolymer with at least onepolyol (i.e., reactant having more than two reactive hydroxyl groups)suitable for cross-linking of the prepolymer to provide an elastomericpolyurethane as described herein. Such a polyol typically has amolecular weight from about 100 to about 500 (preferably from about 200to about 400) although those of lower or higher molecular weights can beused in some instances. Such polyols can be either straight-chain orcyclic. Triols are commonly preferred to avoid excessive cross-linkingwhich would interfere with desired elasticity of the polyurethane. Ingeneral, such a polyol is employed in an amount capable of reacting withfrom about 90% to about 130% of the isocyanate end groups in theprepolymer used in preparation of the desired polyurethane. Numerousexamples of such polyols, as well as polyester and polyether monomersand isocyanates useful in preparation of the afore-mentionedprepolymers, are disclosed in the published literature and may bereadily found by reference thereto including, e.g., the aforementionedU.S. Pat. No. 3,769,265, the disclosure of which is incorporated hereinby reference, and "Polyurethanes--Chemistry and Technology", J. H.Saunders and K. C. Frisch, Interscience Publishers, Division of JohnWiley and Sons, Parts I and II (1962 and 1964); "The Development and Useof Polyurethane Products", E. N. Doyle, McGraw-Hill Book Co. (1971); and"A Glossary of Urethane Industry Terms", S. Alan Stewart, The MartinSweets Co., Inc., Louisville, Ky. (1971).

As aforesaid, it has been found in accordance with this invention thatundesired hardening of an elastomeric polyurethane, e.g. while in an enduse having a required degree of elasticity (a maximum hardness) or instorage prior to such use, can be inhibited by preparing thepolyurethane from polyurethane-forming reactants in admixture with apolyurethane-plasticizing composition comprising a carboxylate having atleast two monocyclic aryl radicals per molecule and a molecular weightfrom about 200 to about 1,000. Typically, but not necessarily, thecarboxylate contains no more than two of such aryl radicals. Alsopreferably, the carboxylate has a molecular weight from about 250 toabout 500. Carboxylates especially preferred for such use include benzylphthalates, e.g. C₂ -C₁₂ alkyl benzyl phthalates such as butyl benzylphthalate, heptyl benzyl phthalate, octyl benzyl phthalate, nonyl benzylphthalate and texanol benzyl phthalate. (Texanol is2,6,6,8-tetramethyl-4-oxo-3-oxo-nonan-7-ol.) Also advantageously usefulas such urethane-plasticizing carboxylates are various dibenzoates suchas, e.g., di(propylene glycol)dibenzoate or di(ethyleneglycol)dibenzoate. In most embodiments, such a carboxylate is includedin the polyurethane-forming reactant mixture in an amount from about 10to about 100 (preferably from about 20 to about 60) parts per 100 partsby weight of the end-capped prepolymer and cross-linking polyol in themixture.

Also included in such a composition in accordance with this invention isa glycol (i.e., a dihydroxy compound otherwise called a diol) having amolecular weight from 62 to about 500, e.g. ethylene glycol, a propyleneor butylene glycol, diethylene or dipropylene glycol or a pentane orhexane diol. Usually, for better compatibility with the carboxylateplasticizer, the glycol employed preferably has a molecular weight of atleast about 76, and even more desirably at least about 90. Generally,e.g. for reasons of cost, it is also desirable to use a glycol having amolecular weight not higher than about 250, and even more advantageouslynot higher than about 150. Also preferably, the glycol is an alkyleneglycol (even more preferably an α,ω-alkylene glycol) such as, e.g.,1,3-propylene glycol, 1,4-butylene glycol, 1,6-hexane diol, dipropyleneglycol or, typically most preferred, diethylene glycol. (As used herein,the term "alkylene glycol" embraces oligomeric alkylene glycols such as,e.g., diethylene or dipropylene glycol, as well as monomeric alkyleneglycols such as, e.g., ethylene glycol or 1,4-butane diol.)

Normally, the glycol should be present in the compositions of thisinvention in an amount, based on the weight of thepolyurethane-plasticizing carboxylate, from about 0.1% to about 5%, andpreferably from about 0.5% to about 3%. As expressed herein withreference to inclusion in a mixture of polyurethane-forming reactants,the just-mentioned amounts of a glycol should be understood as being inaddition to any glycol contained in the aforementioned polyester orpolyether prepolymer. Expressed otherwise, such amounts of glycol referto those added to the aforementioned compositions as free (unreacted)glycol, as distinguished from glycol that has been effectivelyneutralized, e.g. by reaction with an acid to form a polyester or bypolymerization to form a polyether.

In preparation of elastomeric polyurethanes as described herein theprepolymer, cross-linking polyol, carboxylate plasticizer and glycol canbe combined in any convenient sequence. For best results, however, andparticularly with respect to inhibition of hardness of the resultingpolyurethane, it is preferred that the prepolymer, carboxylate andglycol be combined (generally at elevated temperature) before additionof the polyol to the reaction mixture.

As aforesaid, the process improvements and compositions of thisinvention inhibit the tendency of elastomeric polyurethanes, asdescribed herein, to undergo the gradual, undesired hardening which hasbeen heretofore a major drawback to large-scale commercial use of suchpolyurethanes. In many uses, e.g. printing rollers, a hardening of about10% or more as measured in terms of Shore A Hardness is considered aserious problem in that it substantially impairs the suitability of thepolyurethane for such uses. The present invention, however, has beenfound to limit increases in such polyurethane hardness over extendedperiods of time to percentages substantially lower than 10% and, in someembodiments, to maintain the polyurethane hardness essentially constantover extended time periods.

EXAMPLE I

40 parts of a homogeneous plasticizer composition consisting of 99%butyl benzyl phthalate and 1% ethylene glycol and 100 parts ofpolyurethane prepolymer were separately warmed to 66° C. and thenstirred together. The prepolymer, classified as an 80-durometer(unplasticized) resin, was a polyester of reactants consistingessentially of ethylene glycol and adipic acid, end-capped with toluenediisocyanate and having an average molecular weight between 3,500 and4,000 and an average number of isocyanate groups per molecule of 3.25.The blended mixture was then heated in an oven to 93° C. 6.9 parts of aconventional polyurethane cross-linking triol having a molecular weightof approximately 270, an OH content of 18.8%, and OH number of 620, aviscosity of 750 cps and a 1.15 specific gravity was blended into themixture which was then deaerated under essentially full vacuum (5 mmHg.) for 10 minutes in a 93° C. oven, cured for two hours at 149° C. andthen post-baked for 16 hours at 100° C. Shore A Hardness of theresulting elastomeric polymer was 24. It rose to 25 (a 4% increase)after two days of storage under standard conditions (22±1.5° and 50±5%relative humidity) but then remained constant at that level throughout aperiod of six months under those conditions.

COMPARATIVE EXAMPLE A

When the procedure of Example I was essentially duplicated except thatthe plasticizer composition was 100% butyl benzyl phthalate, Shore AHardness of the resulting polyurethane increased 9% within the firstthree days and remained essentially constant at that level.

EXAMPLE 2

When the procedure of Example I was essentially duplicated except thatthe plasticizer composition consisted of 98.3% butyl benzyl phthalateand 1.7% diethylene glycol, Shore A Hardness of the resulting polymer(approximately 25) remained unchanged throughout a period of six months.

COMPARATIVE EXAMPLE B

When the procedure of Example II was essentially duplicated except thatthe plasticizer was 100% di(methoxyethyl)phthalate, Shore A Hardness ofthe resulting polyurethane increased substantially within several daysto a degree considered unattractive for use of the polyurethane inprinting roller covers, and remained essentially constant at that level.

What is claimed is:
 1. An elastomeric polyurethane composition preparedby subjecting to polyurethane-forming reaction conditions an elastomericpolyurethane-forming reactant mixture comprising (a)diisocyanate-capped, essentially linear polyester or polyether, (b)polyol suitable for cross-linking of said polyester or polyether toprovide an elastomeric polyurethane, said polyol having more than tworeactive hydroxyl groups, (c) polyurethane-plasticizing carboxylatehaving a molecular weight from about 200 to about 1,000 and at least twomonocyclic aryl radicals per molecule and, based on the weight of saidcarboxylate, from about 0.1% to about 5% (d) glycol having a molecularweight from 62 to about
 500. 2. Composition of claim 1, said polyesteror polyether having an average molecular weight from about 500 to about20,000, said polyol having a molecular weight from about 100 to about500 and said carboxylate having a molecular weight from about 250 toabout 500 and no more than two aryl radicals per molecule. 3.Composition of claim 2, said glycol consisting essentially of alkyleneglycol having a molecular weight from about 76 to about
 250. 4.Composition of claim 3, said mixture containing from about 10 to about100 parts of said carboxylate per 100 parts by weight (a) plus (b). 5.Composition of claim 4, said (a) consisting essentially of polyesterreaction product of α,ω-alkylene glycol containing from two to about sixcarbon atoms per molecule and α,ω-alkylene dicarboxylic acid containingfrom about four to about eight carbon atoms per molecule.
 6. Compositionof claim 5 wherein said polyester has an average molecular weight fromabout 1,000 to about 6,000 and comprises reaction product of ethyleneglycol and adipic acid.
 7. Composition of claim 5, said (d) consistingessentially of α,ω-alkylene glycol having a molecular weight from about90 to about
 150. 8. Composition of claim 7, said (d) consistingessentially of diethylene glycol and said composition containing fromabout 0.5% to about 3% of said diethylene glycol, based on the weight ofsaid carboxylate.
 9. In a process for preparing an elastomericpolyurethane by subjecting to polyurethane-forming reaction conditionsan elastomeric polyurethane reactant mixture comprising (a)diisocyanate-capped, essentially linear polyester or polyether, (b)polyol suitable for cross-linking of said polyester or polyether toprovide an elastomeric polyurethane, said polyol having more than tworeactive hydroxyl groups, and (c) plasticizer for said polyurethane, theimprovement which comprises using as said plasticizer a carboxylatehaving a molecular weight from about 200 to about 1,000 and at least twomonocyclic aryl radicals per molecule and including in said mixture fromabout 0.1% to about 5%, based on the weight of said carboxylate, of (d)glycol having a molecular weight from 62 to about
 500. 10. Processimprovement of claim 9, said polyester or polyether having an averagemolecular weight from about 500 to about 20,000, said polyol having amolecular weight from about 100 to about 500 and said carboxylate havinga molecular weight from about 250 to about 500 and no more than two arylradicals per molecule.
 11. Process improvement of claim 10 wherein (a),(c) and (d) are combined before (b) is added to said mixture. 12.Process improvement of claim 11, said glycol consisting essentially ofalkylene glycol having a molecular weight from about 76 to about 250.13. Process improvement of claim 12, said mixture containing from about10 to about 100 parts of said carboxylate per 100 parts by weight (a)plus (b).
 14. Process improvement of claim 11, said (a) consistingessentially of polyester reaction product of α,ω-alkylene glycolcontaining from two to about six carbon atoms per molecule andα,ω-alkylene dicarboxylic acid containing from about four to about eightcarbon atoms per molecule.
 15. Process improvement of claim 14, saidpolyester comprising reaction product of ethylene glycol and adipicacid.
 16. Process improvement of claim 14, said polyester having anaverage molecular weight from about 1,000 to about 6,000 and said (d)consisting essentially of α,ω-alkylene glycol having a molecular weightfrom about 90 to about
 150. 17. Process improvement of claim 16 whereinfrom about 0.5% to about 3% diethylene glycol is included in saidmixture.
 18. Composition consisting essentially of butyl benzylphthalate and, based on the weight of said phthalate, from about 0.1% toabout 5% alkylene glycol having a molecular weight from about 76 toabout
 250. 19. Composition of claim 18 consisting essentially of saidphthalate and, based on the weight of said phthalate, from about 0.5% toabout 3% glycol, said glycol consisting essentially of α,ω-alkyleneglycol having a molecular weight from about 90 to about
 150. 20.Composition of claim 19, said glycol consisting essentially ofdiethylene glycol.